A sudden lapse of memory or judgment usually the result of ass gnomes dwelling in your mind cave.

Person 1: Hey baby

Person 2: What!?
Person 1: Oh sorry I thought you were my girlfriend
Person 2: what im a guy!
Person 1: uh errr (realizes it is unmistakably a guy)
Person 3: Don't get upset, it appears he is having a "brain fart"